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No power, no cooling

My Air Conditioner Won't Turn On

Silence when the system should kick on is frustrating, but a dead AC often turns out to be a thermostat, breaker or safety switch issue.

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Technician testing the wiring inside an open outdoor air conditioner

Quick answer

When an AC will not start at all, the usual causes include drained thermostat batteries, a wrong mode, a breaker that tripped, an outdoor disconnect left off, a float switch reacting to a backed-up drain, or a failed control board or capacitor. Several of these can be checked safely in a few minutes. If power and settings are fine, an electrical or control fault is likely.

What may be going on

Possible causes

  1. Thermostat issues

    A blank screen, dead batteries, a schedule that overrides the setting, or a mode left on Off will keep the system from calling for cooling.

  2. Tripped breaker

    A power surge or an overloaded circuit can trip the breaker for the indoor or outdoor unit, cutting power without any obvious sign.

  3. Switch turned off

    The indoor unit often has a switch that looks like a light switch, and the outdoor unit has a disconnect box. Either one can get bumped off.

  4. Tripped float switch

    If the condensate drain backs up, a safety float switch shuts the system down to prevent water damage.

  5. Failed control part

    Power can reach the equipment and still go nowhere if a transformer, control board, contactor or capacitor has died.

Safe checks you can make

  • Make sure the thermostat is set to Cool and the setpoint is several degrees below room temperature.
  • Replace thermostat batteries and check whether a schedule or hold setting is overriding your choice.
  • Look for a tripped AC or furnace breaker in the panel and reset it once.
  • Check the switch near the indoor unit and the disconnect box by the outdoor unit to confirm both are on.
  • Look at the condensate drain area for standing water that might have triggered a float switch.

What not to do

  • Don't reset a breaker over and over. Trip once, reset once; a second trip means a fault.
  • Don't open the furnace or air handler panels, or the outdoor unit's electrical cover.
  • Don't bypass or tape down a safety switch to force the system on.

When to call a professional

If the thermostat responds, the breakers and switches are on, and there is no standing water, yet nothing happens, the fault is likely in the controls or wiring. That kind of testing needs a technician with the right meters.

Spring storms in the Birmingham, AL region can damage electrical parts through surges. Call to get connected with an independent local HVAC provider who can check the system, including the thermostat and its wiring.

  • The breaker trips again right after one reset.
  • The thermostat has power but the system never responds.
  • Water sits in the drain pan and the system will not restart.
  • A burnt smell comes from the indoor or outdoor unit.

What an HVAC provider may inspect

  • Thermostat signals and low-voltage wiring
  • Transformer and control board operation
  • Capacitor and contactor at the outdoor unit
  • Float switch and condensate drain condition
  • Wiring damage from storms, pests or age

Keeping it from happening again

Fresh thermostat batteries each spring, a clean condensate drain and a yearly look at electrical parts during maintenance help a system start reliably. If the AC starts but runs without cooling, see the page on an AC that runs but won't cool, and for an outdoor unit that stays silent while the inside fan runs, see AC blowing warm air.

Where to go next

Related serviceAC Repair

Frequently asked questions

AC Not Turning On: questions homeowners ask

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Some systems have a short built-in delay after power returns, so wait several minutes. If it still won't start, check the breakers, since a surge when power comes back can trip them. A system that stays dead after that may have a damaged electrical part.

Many wired thermostats still use batteries to keep the display and settings working. Weak batteries can cause a blank screen or a thermostat that stops sending signals, so replacing them is a quick first step.

Yes. Many systems have a float switch that shuts everything down when water backs up in the drain pan or line. Clearing the cause usually lets the system restart, though a provider should clear clogs inside the unit.

Still not sure what's wrong?

Call and describe what you're seeing. You'll be connected with an independent HVAC provider serving homes in the Birmingham, AL region.

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